Continuously-variable, solid-angle filter

ABSTRACT

A pair of coaxially mounted, cylindrical, neutral-density filter members geared together for counterrotation, one member being of linearly increasing density clockwise and the other member being of linearly decreasing density clockwise. An arrangement similar to (1) in which parallel linear filter members replace the coaxial cylindrical filter members. A rheostat-controlled light source, a slit mask, a pair of movable beds, gears for driving the beds past the slit at prescribed speeds, and a pair of light-sensitvve film strips respectively mounted on the beds for making the filter members. An arrangement of the filter members in a spectral zonal color reconnaissance viewer.

[ Oct. 30, 1973 1 CONTINUO LY- ARIABLE, Primary Examiner-Louis P. Prince Assistant Examiner-A. J. Mirabito Attorney-Eliot S. Gerber SOLID-ANGLE FILTER [75] Inventor: Edward F. Yost,,Ir., Northport,

Assignee: Spectral Data Corp., Hauppauge.

ABSTRACT A pair of coaxially mounted, cylindrical, neutral- 1969 density filter members geared together for counterro- Appl. No.: 871,481

[22] Filed:

tation, one member being of linearly increasing density clockwise and the other member being of linearly decreasing density clockwisev Related U.S. Application Data Continuation-in-part of Ser. No. 627,539, March 31 1967, abandoned.

An arrangement similar to (1) in which parallel linear filter members replace the coaxial cylindrical filter members.

G03b 33/06 G03]J 21/00 A rheostat-controlled light source, a slit mask, a pair [58] Field of [51] Int.

of movable beds, gears for driving the beds past the slit at prescribed speeds, and a pair of light-sensitvve film strips respectively mounted on the beds for making the filter members.

References Cited UNITED STATES PATENTS An arrangement of the filter members in a spectral zonal color reconnaissance viewer.

353/31 1 Claim, 10 Drawing Figures 353/31 FOREIGN PATENTS OR APPLICATIONS 3,041,924 Strass..... 2,909,097 Alden 3,449,045 6/1969 11/1940 Germany 353/31 Patented Oct. 30, 1973 4 Sheets-Sheet l INVENTOR. EDWARD F. YOST, JR.

his 1 ATTORNEYS Patented Oct. 30, 1973 4 Sheets-Sheet r:

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EDWARD F. YOST, JR.

BY 5 4% F Gwwa&

his ATTORNEYS Patented Oct. 30, 1973 4 Shea ts SheeL :5

I2 8%?) ME 120 INVENTOR. EDWARD F. YOST, JR.

his ATTORNEYS Patented Oct. 30, 1973 4 Sheets-Sheet 4 omN o Swim 2:

omN v2 INVENTOR EDWARD F. YOST, JR. BY

his ATTORNEYS CONTINUOUSLY-VARIABLE, SOLID-ANGLE FILTER CROSS REFERENCE TO RELATED APPLICATION This is a continuation-in-part of my copending application Ser. No. 627,539, filed Mar. 31, 1967, for Continuously Variable, Solid-Angle Filter, now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to filters and, more particularly, to a novel and highly-effective filter adapted to filter radiation (a) over a solid angle, (b) in a continuously, rather than discretely variable manner, and (c) without changing the frequency dependence of energy in the filtered radiation as a function of intensity of the filtered radiation: i.e., in the case of visible radiation, without changing the color temperature of the filtered radiation. The invention relates also to a novel and highly-effective arrangement of the filter members in a spectral zonal color reconnaissance viewer.

2. The Prior Art Means are known for filtering radiation such as light over a solid angle and are disclosed, for example, in my copending application, Ser. No. 519,854, filed Jan. 1 1, 1966, for Spectral Zonal Color Reconnaissance System. Such conventional filters have the serious disadvantage, however, that they are not continuously variable as to density. On the contrary, it is possible to adjust the density of such filters only in discrete steps. As a result, the desired intensity of light of a given chromaticity is not always available. It is possible, of course, continuously to vary the intensity of the light emanating from the light source by means of a rheostat or equivalent device in the supply circuit for the light source, but this expedient changes color temperature as a function of intensity. It is also possible, when employing a conventional filter of the type referred to above, to decrease the density gap" between one filter and the next, but this expedient results in a cumbersome arrangement having an inordinate number of filter elements and provides only an approximation of the desired control of intensity of the filtered radiation.

SUMMARY OF THE INVENTION An object of the present invention is to remedy the shortcomings noted above. In particular, an object of the invention is to provide a rugged, simple, inexpensive, compact filter whereby the flux of a radiation source is rendered (a) uniform as to a first characteristic such as intensity over a predetermined solid angle, (b) continuously variable as to such characteristic as a function of time, and (c) constant as to a second characteristic, such as color temperature, notwithstanding variations of the first characteristic with time. Another object of the invention is to provide novel apparatus and methods for making the new filter. still another object of the invention is to provide a spectral zonal color reconnaissance viewer wherein the brightness of each of a plurality of projected images of a common scene can be continuously varied independently of the brightness of the other images and without any effect on color temperature.

The foregoing and other objects are attained in accordance with the invention by the combination of first and second filter members each substantially continuously graded as a function of at least one dimension from a first location at which the member has a density or other appropriate quality such that it transmits a maximum value of a first characteristic such as intensity to a second location at which the member has a density or other appropriate quality such that it trans mits a minimum value of such characteristic. The members are mounted so that they transmit radiant flux in series and so that the direction of the first location from the second location on the first member is opposite to the direction of the first location from the second location on the second member. Means are further provided for establishing relative movement of the members with respect to each other. The members may be curved or linear. The new filter members of the invention are made in accordance with the invention by moving them simultaneously and proportionately past means for imparting to them the requisite variation in density or other appropriate quality. The filter members are employed in a spectral zonal color reconnaissance viewer in pairs, one pair being operatively associated with each of the light sources in the viewer.

BRIEF DESCRIPTION OF THE DRAWING An understanding of other aspects of the invention may be gained from a consideration of the following detailed description of representative embodiments of the invention and of the accompanying figures of the drawing, in which:

FIG. 1 is a plan view of a first embodiment of apparatus constructed in accordance with the invention;

FIG. 2 is an axial sectional elevation taken generally along the line 2-2 of FIG. 1 and looking in the direction of the arrows;

FIG. 3 is a graph showing the density and transmission characteristics, as a function of a first dimension, of a filter constructed in accordance with the invention at two different settings;

FIG. 4 is a graph showing the density and transmission characteristics, as a function of a second dimension normal to the first dimension, of a filter constructed in accordance with the invention;

FIG. 5 is a plan view of an alternate embodiment of the invention;

FIG. 6 is a view in elevation of the apparatus of FIG.

FIG. 7 is a plan view of apparatus constructed in accordance with the invention for making the filter elements of the invention;

FIG. 8 is a sectional elevation taken generally along the line 88 of FIG. 7 and looking in the direction of the arrows;

FIG. 9 is a sectional elevation taken generally along the line 99 of FIG. 8 and looking in the direction of the arrows; and

FIG. 10 is a perspective diagram of a spectral zonal color reconnaissance viewer constructed in accordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS inasmuch as this is the presently preferred aspect of the invention. Other characteristics of radiation may, however, be substituted for those about to be considered.

FIG. 1 shows a first preferred embodiment of apparatus constructed in accordance with the invention. A pair of coaxial, complementary cylindrical filter members 12 and 14 are provided. The outer filter member 12 has a density which is neutral and which increases linearly and continuously counterclockwise as seen in FIG. 1 as a function of angular distance around the circumference of the cylindrical filter member 12 from a point 16 of minimum density to a point 18 of maximum density. The low-density and high-density ends of the cylindrical filter member 12 form a junction or splice 20. The inner filter member 14 is also of neutral density and is continuously and linearly graded in a counterclockwise direction as a function of angular distance around the circumference of the cylindrical filter member 14 from a region 22 of maximum density to a region 24 of minimum density. The high-density and lowdensity ends of the filter member 14 form a junction or splice 26. Thus, the outer filter member 12 is of continuously and linearly increasing density counterclockwise, and the inner filter member 14 is of continuously and linearly decreasing density counterclockwise.

The densities of the filter members 12 and 14 are complemental so that the total density of the two filter members 12 and 14 as seen by radiant flux originating at a point on their common axis and traveling along a line normal to their common axis and through the filter members is constant over a given angle of the circumference of the cylindrical filter members for any given relative angular orientation of the filter members.

A given solid angle A of the apparatus is selected as a window through which the radiation to be controlled is filtered. The solid angle is defined by the broken lines 28 and 30 in FIG. 1 and 32 and 34 in FIG. 2. Radiation from a source such as a filament 36 within a light bulb 38 is intercepted beyond the solid angle A by an opaque heat shield 40 which also protects the filter members 12 and 14 from damage by the heat emanating from the light bulb 38. The radiation may also be intercepted by other opaque portions of the apparatus (not shown). Radiation is transmitted, however, by a heat-reflecting window 42 within the solid angle A and by the portions of the filter members 12 and 14 within the solid angle.

Rotational means is provided for counterrotating the cylindrical filter members 12 and 14. The cylindrical filter member 14 is mounted in an annular groove 44 formed in a raised flange 46 on the outer periphery of a generally disc-shaped rotatable plate 48. A downwardly projecting flange 50 on the outer periphery of the plate 48 is slidably received within a groove 52 formed in an annular track member 54 mounted on a base 56.

The plate 48 has a downwardly projecting spindle portion 58 which may be manually grasped in order to rotate the spindle 58, plate 48, and inner cylindrical filter member 12 clockwise or counterclockwise as shown by the double-headed arrow in FIG. 2.

Similarly, the outer cylindrical filter member 12 is mounted for rotation about its axis. The outer cylindrical member 12 is received in a groove 60 formed in an annular flange 62 on an annular member 64 the lower end 66 of which is slidably received in groove 68 formed in an annular track 70 affixed to the base 56.

Annular rack gears 72 and 74 formed on the outer and inner circumferences, respectively, of the plate 48 and the annular member 64 engage a pinion gear 76 mounted on a rotatable shaft 78 mounted in an aperture 79 in the base 56 and secured against translational movement with respect to the base 56.

Those skilled in the art will understand that rotation of one of the cylindrical filter members 12 and 14 in one direction through a given angle causes rotation of the other cylindrical filter member 14 or 112 in the opposite direction through an equal angle. The rotational means includes coarse adjustment means and fine adjustment means. Coarse adjustment of the relative angular orientation of the cylindrical filter members I2 and 14 is provided by manual engagement of the spindle 58. Fine adjustment of the relative angular orientation of the filter members is provided by a bevel gear 80 formed at the lower end of the rotatable shaft 78 which engages a bevel gear 82 formed at the right end (as seen in FIG. 2) of a horizontal shaft 84 extending rotatably through an aperture 85 in the base 56. A knob 86 on the left end (as seen in FIG. 2) of the rotatable shaft 84 is engageable manually for rotating the shaft 84, bevel gears 80 and 82, shaft 78, and pinion 76 and for counterrotation of the plate 48 and annular member 64 and hence of the cylindrical filter members 12 and 14. Motorized control of the spindle 58 and knob 86 can of course be provided if desired. Also, roller or ball bearings can be provided to facilitate operation of the moving parts.

Additional pinion gears similar to the pinion 76 may be provided at points spaced along the circumferences of the rack gears 72 and 74. Once such additional pinion gear 88 is shown. It need not be described in detail inasmuch as its structure and function are evident from the preceding description of the pinion gear 76 and associated structure.

The heat shield 40, window 42, and lamp 38 are mounted on a stationary base plate 90 provided with a depending portion 92.

The depending portion 92 extends within a hollowedout portion of the spindle 5S and guides the spindle 58 and plate 48 and associated structure in rotation. Means (not shown) is provided connected to the heat shield 40 for maintaining the base 90 and structure mounted thereon stationary notwithstanding rotation of the spindle 58.

FIG. 3 shows how the apparatus 10 functions. FIG. 3 plots density along the ordinate as a function of circumferential distance around the filter members 12 and I4 expressed in radians. A first graph 94 plots the density of the outer filter member 12. As the graph shows, the density increases from a minimum value which is arbitrarily set at zero linearly to a maximum value which may be arbitrarily called ll. Along the same angular distance (but a lesser circumferential distance, inasmuch as the inner filter member 14 is of smaller di ameter than the outer filter member 12), the density of the inner filter member 14 decreases, as shown by a graph 96, from a maximum of T at zero radians to a minimum of zero at 211' radians. FIG. 3 is related to FIG. 1 by the convention that, in polar co-ordinates, the zero direction for angular measurement is from the origin (the filament 36) horizontally to the right (in FIG. l) and increasing angles are measured counterclockwise (in FIG. ll).

If it is desired that the solid angle A measure 11 radians around the circumference of the filter members 12 and 14, which of course is a larger angle than that shown as constituting the window in FIG. 1, the circumferential limits of the solid angle A may be represented by vertical lines 98 and 100 respectively at 1r/2 and 37r/2 radians in FIG. 3. Between the vertical lines 98 and 100, for the setting of the apparatus represented by the solid graphs 94 and 96, the total density of the filter members 12 and 14, and hence the total attenuation of light passing in series therethrough, within the angle A, is constant. The total density is represented thus by a horizontal dotted line 102.

In particular, the density at 1r/2 radians of the filter member 12 as represented by the graph 94 at the point 104, plus the density of the filter member 14 at 1r/2 radians as represented by the point 106 on the graph 96, equal a value defined by the horizontal line 102. The same is true at all other points along the abscissa, inasmuch as the curves 94 and 96 are both linear, and the slope of the curve 94 is equal but opposite in sign to the slope of the curve 96.

Consider next the case where the filter members 12 and 14 are displaced in opposite directions, each through an angle 1r/2 radians, from the positions represented by the solid curves 94 and 96. The filter member 12 has been rotated counterclockwise through an angle of 90 (as seen in FIG. 1), so that the junction or splice is in a position at the top of FIG. 1 and the curve 94 is displaced to the right in FIG. 3 through one quadrant to a position 94. Similarly, the inner filter member 14 has been rotated clockwise (as seen in FIG. 1) through an angle of 90 so that the splice 26 is at the bottom of FIG. 1 and the curve 96 of FIG. 3 has been displaced to the left through one quadrant to a position 96'.

In this relative angular orientation of the cylindrical filter members 12 and 14, the density or attenuation of radiation passing in series through the filter members is again constant over the angle A, whatever the angle may be (shown in FIG. 3 as extending between 1r/2 and 31r/2 radians) and is at a value 102' which is less than the value 102.

In accordance with some methods of making the filter members 12 and 14, there may be density variations between the upper and lower edges of the filter members (as seen in FIG. 2). Thus, as a curve 108 in FIG. 4 shows, one of the filter members may be of increasing density in a direction parallel to the axis of the circumferential filter members 12 and 14 and, as a curve 110 in FIG. 4 shows, the other filter member may be of decreasing density in the same direction along the axis. These variations in density are compensated by mounting the filter members so that the increasing density of one filter member is opposed, as shown by a graph 108', to the decreasing density of the other filter member shown by the graph 1 10. Thus, along the axis of the circumferential filter members as well as around the circumference thereof, the attenuation of radiation passing in series through the filters, within the angle A, is constant.

It will be apparent from FIGS. 3 and 4 considered together that, although the attenuation of radiation passing through the filters within the entire solid angle A, regardless of the size of the angle, up to a limit of Zn radians, is (a) constant at a given time and (b) variable in time, there is no change in the frequency dependence of energy in the filtered radiation as a function of the intensity of the filtered radiation.

As a comparison of FIGS. 1 and 3 shows, there is wide latitude in the choice of the size of the solid angle A. In general, the choice of the size of the angle will depend on the desired application. Of course, the larger the angle, the less the difference between the maximum and minimum levels of the attenuation, and, the smaller the angle, the greater the difference.

FIGS. 5 and 6 show an alternate embodiment of apparatus constructed in accordance with the invention in which linear neutral-density filter members 112 and 114 are substituted for the cylindrical members 12 and 14. As FIG. 6 shows, the linear member 112 is of linearly and continuously increasing density towards the left of the figure and the linear filter member 1 14 is of linearly and continuously increasing density towards the right of the figure. The members may be flexible film members provided with sprocket holes 116 and 118, respectively, and adapted to be wound about spools 120 and 122, respectively mounted on shafts 121 and 123. Pinion gears 124 mounted on shafts 126 are provided for assuring countermovement of the filter members 112 and 114 through equal distances when either is moved by suitable means (not shown). The shafts 126 are secured by suitable (not shown) against translational movement. The source (not shown) of radiation to be filtered is mounted to project radiant flux in series through the filter members 112 and 114.

Since the linear filter members may be made in any length, there is no necessary relation between the size of the solid angle of the filtered radiation and the difference between the maximum and minimum levels of the attenuation. The solid angle is limited, of course, in this embodiment, to a value less than 180. In another embodiment (not shown), however, similar to the embodiment of FIGS. 5 and 6, the flexible filter members 112 and 114 are trained past the soruce of radiation to be filtered in circular arcs, and the solid angle of the filtered radiation can exceed l80.

The filters of the present invention may be made in a number of ways. For example, graded vacuum deposition of a thin metallic film may be employed to achieve the desired density values. Again, etching of glass in an acid bath by movement at a variable rate through the bath, and impregnation of the etched sur'- face with a substance such as carbon black may be employed. In another method, lines may be ruled on glass in such a manner as to achieve variable separation as a function of distance or angle. Again, lithographic process may be employed to place variable spaced dots on the filter members. Or, a paper mask may be made using an electrostatic grid and rotating the paper at variable speed beneath it. The mask can be later used for spraying a deposit of dots on the glass cylinder to form the requisite density gradient.

A preferred method of making the filter members photographically for the embodiment of FIGS. 1 and 2 is shown in FIGS. 7-9.

FIG. 8 shows a light source 128, a lens 130 for projecting light from the light source 128, and a mask 132 formed with a slit 134 therein permitting a narrow beam of light from the source 128 to pass through the slit 134 and impinge on film strips or members 136 and 138 rendered light-sensitive by the provision of photographic emulsions facing the light source 128. The film strips are secured by cover glasses 136' and 138, respectively. The light-sensitive film strips 136 and 138 are mounted in the same plane in beds 140 and 142, respectively. The beds 140 and 142 are formed with racks 144 and 146, respectively, on their lower sides. The racks 144 and 146 are at different elevations and are geared to pinion gears 148 and 150, respectively, of different diameter. The pinions 148 and 150 are fixed on a shaft 152 which is secured against translational movement by bearings 154 and adapted to be rotated by a pulley 156 connected by a belt 158 to a pulley 160 on a rotatable shaft 162 of drive means such as a small electric motor 164. A cam 166 fixed on the shaft 152 has a projection 168 engaging a cam follower 170 controlling a rheostat 172 provided with leads 174 in the circuit of the light 128.

Left-hand ends 176 and 178 of the film strips 136 and 138, respectively, shown in FIG. 7 are initially aligned under the slit 134 and the film strips are then moved to the left (as seen in FIG. 7) under the slit 134. The film strips 136 and 138 are illuminated only through the slit 134 and move at different speeds, the strip 138 moving faster because it is driven by a larger pinion gear 150. The ratio between the diameters of the pinion gears 148 and 150 is the same as the ratio of the lengths of the portions of the film strips 136 and 138 to be used in making the filter members, so that the right-hand edges (not identified by reference numeral) of such portions of the film strips arrive simultaneously at the slit 134. The portion of the strip 138 used in making the outer filter member 12 is of course longer than the portion of the strip 136 used in making the inner filter member 12.

During the movement of the film strips 136 and 138, the cam 166 rotates proportionately and controls the rheostat 168 to adjust continuously the intensity of the light from the source 128. The gamma characteristic of the film and the light output of the source as a function of supply voltage thereto may be taken into consideration and compensated by the design of the cam 166 or rheostat 172 so that the optical density of the developed film varies continuously and quite linearly. After the film strips 136 and 138 are developed and dried, the strip 136 is curved cylindrically and preferably mounted between two closely fitting glass cylinders to form the inner filter member 14, and the strip 138, being longer than the other, is similarly treated to form the outer filter member 12. Because the strip 138 moves more rapidly past the slit 134 than does the strip 136, the exposure of the former is somewhat less than that of the latter. This relationship between exposures is desired in order to compensate for the fact that a correspondingly greater area of the outer filter member 12 lies within the angle A (see FIG. 1).

Because the orthographic projection of the light source 128 onto the plane of film strips 136 and 138 lies between the strips 136 and 138, the edges of the strips 136 and 138 adjacent to each other are optically denser after development than the edges remote from each other, there being a gradient as shown in FIG. 4. This density gradient is compensated as also shown in that figure. Geometric considerations of course require that, in cylindrically curving the film strips 136 and 138 exposed by the apparatus of FIGS. 7-9, one strip be curved toward the emulsion side and the other away therefrom, in order to attain simultaneously the relationships shown in NOS. 3 and 4.

In making the linear filter members shown in FIGS. 5 and 6, the racks 144 and 146 are preferably at the same elevation and the gears 148 and of the same size, inasmuch as translational movement in opposite directions of the filter members is equal in that embodiment.

Another method in accordance with the invention is similar to that described above, but the means for grading the density of the filter members depends upon vacuum deposition of a thin metallic film through a slit mask, the emanations from the source impinging on the filter members for changing their optical density being molecules rather than photons.

FIG. 10 shows schematically and in perspective a spectral zonal color reconnaissance viewer 180 employing projection filters 182, 184, 186 and 188 and desaturation filters 190, 192, 194 and 196 in accordance with the invention. The filters are shown as being cylindrical, but any of the other embodiments of filters in accordance with the invention can be substituted for the ones shown in FIG. 10. Each of the filters 182-196 is of the composite design described above.

A separate light source is operatively associated with each of the filters 182-196. That is, a light source 202 is associated with the filter 182; a light source 204 is associated with the filter 184; a light source 206 is associ ated with the filter 186; a light source 208 is associated with the filter 188; a light source 210 is associated with the filter 190; a light source 212 is associated with the filter 192; a light source 214 is associated with the filter 194; and a light source 216 is associated with the filter 196.

A plurality of representations 218, 220, 222 and 224 of a common scene is recorded on a strip of photographic film 226 held flat by glass platens 228 and 230, and wound on a pair of spools one of which is shown at 232. Each representation is a gray-scale photographic recording made at the same time and from the same perspective in response to reflection from the subject of interest, primarily in different regions of the electromagnetic spectrum. Various techniques of spectral zonal photographic analysis are adapted to extract a broad range of data regarding the subject, and the apparatus of FIG. 10 facilitates the process.

Each of the representations 218, 220, 222 and 224 is operatively associated with a different one of four sets of light means. The first set of light means, operatively associated with the representation 218, comprises the light sources 202 and 210; the second light means, operatively associated with the representation 220, comprises the light sources 204 and 212; the third light means, operatively associated with the representation 222, comprises the light sources 206 and 214; and the fourth light means, operatively associated with the representation 224, comprises the light sources 208 and 216. The two light sources of each light means control the brightness and saturation of the respective projections of the representations 218, 220, 222 and 224 on a viewing screen 234. The hue of the projections is controlled by four filters 236, 238, 240 and 242, respectively. Each of the filters includes sections of different color, such as sections R, G, and B, which are colored, respectively, red, green, and blue. Projections of the representations 218, 220, 222, and 224 are effected by means of projection lenses 244, 246, 248 and 250, respectively.

The light from the projection lamps 202, 204, 206 and 208 is mixed with the light from the desaturation lamps 210, 212, 214 and 216 by beam-splitting prisms 244', 246', 248' and 250', respectively. These are prisms that transmit light from the projection lamps 202, 204, 206 and 208 through the representations 218, 220, 222 and 224,"respectively, and through the projection lenses 244, 246, 248 and 250, respectively; and that reflect light from the desaturation lamps 210, 212, 214 and 216, respectively, so that it likewise passes through the representations 218, 220, 222 and 224 and through the projection lenses 244, 246, 248 and 250, respectively.

The projection lenses 244, 246, 248 and 250 are oriented so that the projections of the representations 218, 220, 222 and 224 are superimposed on the viewing screen 234 to form a composite image.

The operation of the apparatus of FIG. 10 is as follows: initially, each projection lamp 202, 204, 206 and 208 and each desaturation lamp 210, 212, 214 and 216 is adjusted to a desired level of brightness by adjustment of a rheostat or potentiometer-rheostats 252, 254, 256 and 258, respectively, in the case of the projection lamps 202, 204, 206, and 208; and rheostats 260, 262, 264 and 266, respectively, in the case of the desaturation lamps 210, 212, 214 and 216. During this adjustment, of course, switches 268, 270, 272 and 274, respectively, operatively associated with the rheostats 252, 254, 256 and 258, areclosed; and switches 276, 278, 280 and 282, respectively, operatively associated with the rheostats 260, 262, 264 and 266, are likewise closed. The switches 268282 are connected to a common line 284 that goes to a source 286 of potential such as a battery. Then, with the switches 268-282 opened or closed in accordance with the particular effect desired, the filters 182-l96 are adjusted so that the projections onto the screen 234 of the representations 218-224 are brought to the desired levels of brightness and saturation. The filters 236-242 are likewise adjusted so that the projections of the representations individually have the desired hue.

By manipulation of the adjustments of the filters 182-196 and 236-242 and of the switches 268282, it is possible to extract a wide variety of data regarding the common scene that is the subject of the representations 218224.

Thus, there is provided in accordance with the invention a rugged, simple, compact, inexpensive filter whereby the flux of a radiation source is rendered uniform as to a first characteristic such as intensity over a predetermined solid angle, continuously variable as to such characteristic as a function of time, and constant asto a second characteristic such as color temperature,

notwithstanding variations of the first characteristic with time. i 1

Novel and highly effective methods and apparatus for making the new filter members are also provided. The

invention has application to all fields of lighting where the brightness of a light must be controlled and is particularly useful in radiometric and spectral-zonal photographic processes and in environments where space requirements are stringent. An especially useful application is in a spectral zonal color reconnaissance viewer, in which, in accordance with the invention, the brightness and saturation of images projected for interpretation can be varied without in any way affecting color temperature.

Many modifications of the representative embodiments disclosed herein will occur to those skilled in the art. Accordingly, the invention is to be construed as including all of the embodiments thereof within the scope of the appended claims.

I claim:

1. A spectral zonal photographic method of discovering information about a scene comprising the steps of making black and white representations of said scene at different zones of the actinic electomagnetic spectrum using a multi lens camera respectively illuminating each of a plurality of black and white photographic representations of the scene with each of a plurality of illuminating lights, projecting images of said respective representations on a viewing screen in superimposed relation, filtering the lights respectively illuminating each of said representations using a first set of filter elements associated with the lights so that each of said illuminating lights are respectively of uniform intensity over the area of each of said representations, varying the said first set of filter elements so that the intensity of the illumination of each representation is continuously varied, individually coloring said illuminating lights using color means to impart a desired hue to said projected images, respectively superimposing white desaturating lights on the projected representation to desaturate the color of said projected representations thereby varying the saturation without changing the hue, and filtering each of said desaturating lights using a second set of filters associated with the desaturating lights to vary the intensity of said desaturating lights so that said lights are of uniform intensity over the area of each of said representations, and varying the said second set of filter elements so that the intensity of each desaturating light is continuously varied, whereby the brightness and saturation of each of said projected images are continuously and independently adjustable without alteration of color temperature. 

1. A spectral zonal photographic method of discovering information about a scene comprising the steps of making black and white representations of said scene at different zones of the actinic electomagnetic spectrum using a multi lens camera respectively illuminating each of a plurality of black - and white photographic representations of the scene with each of a plurality of illuminating lights, projecting images of said respective representations on a viewing screen in superimposed relation, filtering the lights respectively illuminating each of said representations using a first set of filter elements associated with the lights so that each of said illuminating lights are respectively of uniform intensity over the area of each of said representations, varying the said first set of filter elements so that the intensity of the illumination of each representation is continuously varied, individually coloring said illuminating lights using color means to impart a desired hue to said projected images, respectively superimposing white desaturating lights on the projected representation to desaturate the color of said projected representations thereby varying the saturation without changing the hue, and filtering each of said desaturating lights using a second set of filters associated with the desaturating lights to vary the intensity of said desaturating lights so that said lights are of uniform intensity over the area of each of said representations, and varying the said second set of filter elements so that the intensity of each desaturating light is continuously varied, whereby the brightness and saturation of each of said projected images are continuously and independently adjustable without alteration of color temperature. 